Technical Field
Embodiments of the invention relate generally to magnetic resonance imaging (MRI). Particular embodiments relate to enhancing the clarity of images produced by diffusion-weighted imaging.
Discussion of Art
In magnetic resonance imaging (MRI), human or other animal tissue is subjected to a uniform magnetic field, i.e., a polarizing field B0, so that the individual magnetic moments of particle spins in the tissue attempt to align with the polarizing field, but precess about the field in random order at their characteristic Larmor frequency. If the tissue is subjected to an RF magnetic field, i.e., excitation field B1, which defines an x-y plane and varies at a frequency near a Larmor frequency of selected particles, the net aligned moment, or “longitudinal magnetization” of those selected particles, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment. After B1 is terminated, the tipped spins “relax” back into the precession defined by B0, and, as a result, produce RF signals. The RF signals may be received and processed to form an image. In order to form a pixelated image for human interpretation, gradient magnetic fields, Gx, Gy, Gz, are applied to localize the tissue response to B1.
Diffusion MRI (or dMRI) is a magnetic resonance imaging method, which facilitates mapping of the diffusion process of molecules, mainly water, in biological tissues, in vivo and non-invasively. Such mapping in turn allows a diagnostician to identify tissue abnormalities. A popular form of dMRI is diffusion weighted imaging (DWI) in which the intensity of each image element, i.e., voxel, reflects the best estimate of the rate of water diffusion at that location. This is significant in that the mobility of water is driven by thermal agitation and highly dependent on its cellular environment. As a result, the hypothesis behind DWI is that findings may indicate early pathologic change. For instance, DWI is more sensitive to early changes after a stroke than more traditional MRI measurements such as T1 or T2 relaxation rates.
The gradients used for DWI, however, can cause hyper-sensitization to subject motion, which leads to destructive phase inconsistencies, such as aliasing, in segmented acquisition modes.
In view of the above, it is desirable to provide apparatus and methods for efficiently unwrapping motion-aliased images, so that diffusion weighted images can be more readily usable. Such apparatus and methods might also be helpful toward correcting for motion artifacts, generally.